Two Sum

题目

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

题意

返回符合题意 two integers 在数组中的位置（从0开始数起）

HashMap 查找元素时复是O（1）

分析

1. create a HashMap
2. scan the whole array, if map containsKey (target - nums[i]) return result , otherwise, put every nums[i] as key and every i as value into HashMap

图解

代码

class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer, Integer> map = new HashMap<>();
        for(int i = 0; i < nums.length ; i++){
            if(map.containsKey(target- nums[i])){
                return new int [] {map.get(target- nums[i]), i};
            }else{
                map.put(nums[i],i);
            }
        }
        return null;
    }
}